The present invention relates to digital information transmission systems and, more particularly, to transceiver apparatus and methods for transmitting and receiving high data-rate data signals.
A continuing trend in the development of digital information transmission systems is to increase the effective bandwidth of transmission media by sending data at a faster data rate. Such transmission media include two-conductor twisted pair unshielded cable, such as utilized in telecommunication circuits but can also include other media types such as flat, twin-axial and coaxial cables. As used herein, a high data-rate for transmitting data signals is at least 60 Mbit/s.
It is the present inventors, experience that there are difficulties encountered in attempting to transmit and receive high data-rate data signals on bandwidth limited conductive transmission media. For example, the present inventors have observed that circuitry known in the art for transmitting and receiving high data-rate data signals is typically constructed using complex configurations of discrete circuit components that are not readily integratable and that entail significant expense.
Attempts to increase the transmission data rate are also restricted by physical limitations o the ability to increase the data transmission rate. For example, an increase in the data transmission rate is accompanied by an increase in the high frequency spectral content of the transmitted signal. Such an increase in the high frequency spectral content can result in unacceptably high electromagnetic emission levels which are caused by common mode components on the transmission media. As also known in the art, high frequency spectral content is also a cause of undesirable crosstalk. Also, the levels of electromagnetic emissions resulting from transmission of digital information are subject to conformance with federal and international standards, for example the standards established by the Federal Communications Commission. Therefore, the data transmission rate can be limited by these considerations.
The emissions associated with the high frequency spectral content of a high data-rate data signal may in general be reduced by any M-ary encoding scheme known in the art, such as modified duobinary encoding. Such a modified duobinary encoding technique is disclosed in U.S. Pat. No. 3,993,953 issued on Nov. 23, 1976. Such encoding schemes, however, require driving a transmission line, such as a two-conductor cable, with a multi-level signal. The requirements to encode a high data-rate signal as a multi-level signal, transmit the multi-level signal, and to receive and decode the multi-level signal, further add to the complexity and expense of the apparatus for transmitting and receiving high data-rate signals.
Another problem associated with transmitting and receiving high data-rate signals over a two-conductor unshielded line or other medium is the known attenuating effect such a medium has on the high frequency components of the transmitted signal. While the above-noted encoding schemes reduce the high frequency spectral content of the transmitted data signal, the transmitted high data-rate signal will nevertheless be subjected to some distortion caused by such attenuation. This distortion in turn can, undesirably, cause errors in the detection of the transmitted data. One technique known in the art that is used in an attempt to compensate for such attenuation and distortion of high data-rate data signals is the implementation of line equalization circuitry at the transmitting or receiving end of the transmission line. Such circuitry typically includes an energy storage component, e.g., a capacitor, for storing energy that is subsequently added to the transmitted or received signal in a manner intended to compensate for the attenuated high frequency component of the transmitted signal. Disadvantageously, the energy added to the data signal from the capacitor corresponds to energy extracted from a previous portion of the data signal and therefore cannot be uniquely adapted to each pulse of the digital signal being transmitted or received.
Also as known in the art, the discharge of energy from such line equalization circuitry, whether at the transmitter or receiver, can cause overshoots at transitions in the data signal being compensated. Such overshoots can cause errors in the detection of the transmitted data at the receiver.